The invention relates to a motor vehicle drive.
The invention relates to a motor vehicle drive containing at least one electric machine, which can be operated as an electric motor for driving a motor vehicle through a drive line; a clutch device with at least one clutch in the drive line between the electric machine and an output for driving the wheels of the motor vehicle.
DE 40 05 623 A1 discloses a motor vehicle drive, with which a motor vehicle can be driven either by a combustion engine or by an electric motor or by both using a clutch device. This is therefore what is referred to as a hybrid drive. The combustion engine and the electric machine act on the same vehicle wheels.
DE 40 04 330 C2 discloses a hybrid drive for a motor vehicle, in which a combustion engine and an electric machine act on the same drive line and can each be coupled to or decoupled from it. The electric machine can be operated either as a drive motor, as a generator or as an electric starter motor for starting the combustion engine.
DE 35 42 059 C2 shows a hybrid drive in which the combustion engine drives the vehicle wheels on one axle, and an electric machine acting as an electric motor drives the vehicle wheels on another wheel axle.
DE 199 05 366 C2 discloses a hybrid motor vehicle drive in which the crankshaft of a combustion engine can be connected to a gearbox input shaft by an engageable and disengageable clutch and the crankshaft is connected in a rotationally fixed manner to the rotor of an electric machine, which can be operated as an electric starter motor or as a generator.
DE 196 39 322 A1 discloses an automatically controlled clutch in the drive line of a motor vehicle with a gearbox, in which different drive settings or gear ratios can be selected by the driver, with an actuating unit that automatically actuates the clutch, and with a sensor system that is used to control this unit and, inter alia, detects actuation of a vehicle brake by the driver, actuation of a control element of the engine by the driver and a selected condition of a drive setting or gear ratio and sets the clutch to a creeping torque if the vehicle brake and the control element are unactuated with the engine running. To make manoeuvring easier, this document also discloses adjusting the automatic clutch to a low transmissible torque, i.e. to a creeping torque when a gear ratio has been set, the engine is running and neither the service brake of the vehicle nor an accelerator pedal or the like used to control the output of the engine is actuated. Here, use is made of the fact that the engines of motor vehicles generally have an effective automatic engine control system which is capable of keeping the engine at a minimum speed (e.g. idling speed) when the accelerator pedal is unactuated, even if the engine is subject to a greater or smaller load. In this case, the motor vehicle can creep during manoeuvring operations. However, the power flow between the input and output of the clutch is interrupted completely when the service brake is actuated, making it necessary to adjust the clutch back to the creeping torque at the end of brake actuation. To avoid this, the document proposes that, if the vehicle brake remains actuated, the clutch set to a creeping torque is only opened after a predetermined delay and/or that the creeping torque is reduced slowly in the case of brake actuation and/or that a reduced clutch torque compared with the creeping torque is set if the vehicle brake is being actuated.
In the case of motor vehicles that have only one or more electric motors as traction drives and in the case of motor vehicles which have an electric motor as a traction drive motor in addition to a combustion engine, the electric motor is often subject to driving situations in which it has to supply high torques when the speed is very low or when the motor vehicle is stationary, e.g. when driving onto a curb, when climbing out of an underground garage, when creeping forwards with a trailer on a hill etc.
In these situations, large heat losses occur both in the at least one electric machine, which is used as an electric motor for the traction drive, and in its power semiconductors, which are nowadays used for such electric machines. While these heat losses are uniformly distributed in the rotor and/or the semiconductors of the electric machine during normal driving of the motor vehicle, the heat builds up in the windings instantaneously supplied with current and in the associated semiconductors of the electric machine when the motor vehicle is stationary or creeping slowly.
As a result, the permitted continuous holding torques of the electric machine are in some cases reduced considerably, depending on design.
This calls into question the utility of the electric drive. Overdimensioning of the electric machine for the required continuous static torque is not desirable for reasons of cost and weight.
It is an object of the invention to obtain the required continuous static torque of the electric machine in the special operating case, in which high traction drive torques have to be supplied at a very low motor vehicle speed or when the motor vehicle is stationary without the need to overdimension the electric machine.
According to the invention, this object is achieved by a control device, with which the at least one clutch can be operated in a torque-transmitting slipping manner to avoid thermal overloading both of the clutch of the clutch device and of the electric machine in the special operating case. In the special operating case, increased waste heat arises in the electric machine compared with normal operation when the speed is very low or the motor vehicle is stationary and the torque of the electric machine is simultaneously high.
The invention thus solves the problem by placing in the drive line between the at least one electric machine and the driven wheels at least one clutch, which is operated in a slowly slipping manner in the abovementioned special operating case, ensuring that neither the electric machine nor the clutch is thermally overloaded. The advantage is that the evolution of heat in the electric machine is distributed uniformly and with sufficiently rapid alternation between all the coils and semi-conductors, with the result that temperature peaks are avoided and the electric motor can supply a high continuous static torque over longer periods.
If the motor vehicle drive contains an automatic transmission with selectable gear ratios, a preferred embodiment of the invention can include the automatic transmission or at least one clutch or holding brake used as a clutch device, which can be operated in a torque-transmitting slipping manner by the control device in the way mentioned. The advantage is that there is no need for clutches in addition to the automatic transmission for the invention. To produce the slight slip required, it may be expedient to operate the clutch in a pulsed manner.
In an automatic transmission with an upstream torque converter and a lock-up clutch for locking up the torque converter, it is possible, according to another preferred embodiment, for the lock-up clutch to be used as the clutch of the clutch device, which can be operated in a torque-transmitting slipping manner by the control device in the manner mentioned above. The advantage is that the invention does not require a special clutch. The lock-up clutch can also be operated in a torque-transmitting slipping manner in combination with one or more selector clutches of the automatic transmission, i.e. both can be operated in this way, in the special case mentioned, being operated at alternate times, for example.
The electric machine serving as the traction drive motor is preferably of the type that has at least two individually excitable electric windings. According to the invention, these windings can be excited alternately by the control device in the special operating case, one winding in each case being unexcited and able to cool down while another winding is excited to produce a driving torque. The advantage is that the continuous alternation of excitation between the windings and semiconductors ensures uniform distribution of the electrically generated heat, and overdimensioning of the electric machine for the special operating case is not required; the waste heat in the clutch device is reduced because a low slipping speed is sufficient for uniform distribution of the heat.
In another preferred embodiment of the invention, the clutch device has a plurality of clutches, which can be operated alternately in torque-transmitting slipping mode by the control device to reduce the waste heat peaks in the individual clutches. In addition or as an alternative, the embodiment can be such that the clutches can be operated together (in parallel or in series) by the control device to reduce the waste heat peaks in the individual clutches. The advantage is that, in the special case, the clutches can be operated alternately and/or together, ensuring a reduction in the waste heat peaks and hence lengthening of the service life of the clutch. It is possible also to use clutches that are not designed for prolonged continuous slipping operation with torque transmission.
The invention can be used for motor vehicle drives in which one or more electric machines are provided as electric motors for the traction drive, and can furthermore also be used in combination with a combustion engine as a traction drive. It is possible for the electric motor and the combustion engine to be operated simultaneously or alternately. The combustion engine and the electric machine can be designed to drive the same vehicle wheels or different vehicle wheels. Provision can furthermore be made to use the electric machine as a starter for the combustion engine.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.